The accurate control of fluid flow in microfluidic devices is key to the ability to perform assays, for example diagnostic immunoassays, in a microfluidic device. Fluid flow can be achieved with actuated or passive microfluidics. Actuated microfluidics control fluid flow using an external power source or pump. In passive microfluidics fluid flow is encoded by the design of the microfluidic device itself, rather than externally applied forces, with fluid flow occurring due to capillary forces. Passive microfluidic devices, sometimes referred to as autonomous capillary systems, are attractive due to their low power consumption, portability and low dead volume.
The optimization of performance of assays conducted in microfluidic channels is dependent on control of the timings for the various steps of the assays. Frequently this is achieved by the incorporation of structural delay elements such as delay loops that take a controlled period of time to fill. Such an approach has numerous disadvantages such as the complexity of the channel design with attendant variation in performance arising from difficulties in achieving a uniform sealing of the channel, a limitation of the number of tests that can be placed within a certain area, by virtue of the space occupied by the delay loops, high flow resistance and viscous drag associated with relatively long channels of small cross-sectional area, and the high surface area:volume ratio that characterizes such a design which leads to inefficient clearance of reagent along the channel length.
It is therefore desirable to provide a device with a simplified structural design to overcome problems such as those mentioned above, but in which fluid flow can be controlled to an extent sufficient to allow optimization of assays performed therein.
It has now been determined that these problems can be overcome by control of fluid filling by chemical means, in particular by incorporating within the path of fluid flow in a device deposits of reagents that decrease or increase the rate of fluid flow as they are contacted by a flowing fluid front.